Electron gun



July '28, 1942. B 'Q GARDNER 2,291,462

ELEGTRON GUN Filed July 29, 1940 fzlafflff INVENToR c GARDyER Patented July 28, 1942 STATES PATET OFFICE ELECTRON GUN Application July 29, 1940, Serial No. 348,076

(Cl. Z50-162) 3 Claims.

This invention relates to electron guns for use in cathode ray tubes, and more particularly to an improved anode for use in such electron guns.

Electron guns of the type used heretofore comprise a cathode, a control grid, a rst anode, and a second anode. The rst anode usually has the form of a conductive thin-walled cylindrical sleeve with several apertures spaced at intervals on the axis of the system. The function of these apertures is further to confine the spread of the electrons, the angle of which is initially limited by the aperture in the control grid and the potential applied thereto. The first anode is operated at a positive potential with respect to the cathode, and the potential of the second anode is considerably more positive with respect to the cathode.

As is well understood in the art, this type of electron gun comprises two electron lenses. The rst lens is composed of the cathode surface, the control-grid aperture, and the first aperture of the first anode and their respective potentials. The function of the first electron lens is to cause the electrons emitted by the cathode to converge at a crossover point on the axis of the system of electrodes and shortly in front of the cathode surface. The area of the electron beam at the crossover point is considerably less than the emitting area of the cathode itself. The second electron lens serves to produce an image of the crossover on the viewing screen of the cathode ray tube. This lens is formed by the electrostatic iield at the portion of the electron gun at which the edges of the first and second anodes meet.

Although many diierent types of unipotential iirst anodes have been employed in the past in an effort to improve the concentration of the electron beam, the results have not been entirely satisfactory. Such anodes are relatively complicated in construction.

The object of the present invention, therefore, is to provide an improved first anode for use in an electron gun, which is simple in construction but nevertheless capable of providing substantially better control of the electron beam.

In accordance with the present invention, there is provided in an electron gun a means for concentrating the iiow of electrons. This means comprises an anode extending in the direction of the electron flow and is provided with a passage- Way for the electrons. The anode has a substantial electrical resistance and means are also provided for developing a voltage gradient across the anode in the direction of the electron flow.

In this manner an electrical field is developed in the path of the electron flow which serves to concentrate the electrons therein.

For a better understanding of the invention, together with other and further objects thereof, reference is made to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the accompanying drawing:

Fig. 1 is an elevation, partly in section, of the electron gun portion of a cathode ray tube employing an anode in accordance with the present invention;

Fig. 2 is a modified form of anode adapted for use in the electron gun of Fig. l; and

Fig. 3 is another modified form of anode adapted for use in the device of Fig. 1.

Referring now more particularly to Fig. l of the drawing, there is shown an envelope l containing an electron gun comprising a heater 2, an electron-emissive cathode 3, a control grid 4 having an aperture 5 of conventional design, and a rst anode E of tubular shape and substantial electrical resistance. Adajacent the first anode 6, there is provided a second anode 1 preferably comprising a conductive coating on the inner wall of the envelope I. A source of potential 3, having the polarity indicated in the drawing, is connected by means of the leads 9 and l0 to the ends of the anode 6 for the purpose of developing a voltage gradient thereacross. The inner wall of the anode 6 is cylindrical, the outer wall being conically shaped to give the desired variation in cross-sectional area.

In operation, a potential drop is developed along the longitudinal axis of the anode 6 by means of the potential source 8. By proper adjustment of this potential drop, the effect of the anode upon the electron beam may be altered within wide limits. The lower end of the anode 6 is operated at a positive potential with respect to the cathode 3, While the anode 1 is operated at a considerably higher positive potential. These positive potentials are obtained from any suitable source, not shown. The control grid 4 has applied thereto a suitable negative bias potential and a signal voltage in the conventional manner, from a source which is not shown. Due to the decreasing cross-sectional area of the anode 6 in the direction of electron travel, the electrical resistance per unit length of the anode increases so that the potential gradient near the lower end of the anode 6 is substantially less than it is near the upper end.

Under these conditions of operation, the electrical field which is produced by the anode 6 is indicated approximately by the equipotential lines Il. This electrical eld is such that the concentration of the electrons passing through the aperture 5 of the control grid 4 is substantially increased,

In the modied form of anode l2 shown in Fig. 2, the inner Wall I3 is conically shaped in order to provide a desired electrical eld. The outer Wall lll is tapered sufliciently to provide the proper variation in cross-sectional area along the longitudinal axis 0I the anode. It will be understood that a greater change in electrical resistance per unit length, and hence in the potential gradient, could be obtained by more sharply tapering the outer Wall I4.

Fig. 3 shows an anode l5 having cylindrical inner and outer Walls. In this case, variation in the electrical resistance per unit length is secured by varying the specific resistance of the anode material. If the anode is made of a single substance, this effect may be secured by more highly compressing the material near one end of the anode than near the other end, thus utilizing the well-known phenomenon that the specic resistance of a material decreases as its density is increased. If the anode is formed of a mixture of a conducting or semi-.conducting substance and an insulating material, the specific resistance may be increased by increasing the proportion of the insulating material. apparent that this method of varying the electrical resistance per unit length of the anode is the equivalent, so far as its effect upon the potential gradient and hence upon the electrical eld produced is concerned, of the varying crosssectional area employed in the embodiment of Figs. 1 and 2.

The anode in accordance with the invention may be made of any suitable material having a relatively high specic resistance, such as a mixture of carbon and a ceramic binder. In a preferred embodiment employing this type of anode material, the source 8 of Fig. l may supply a potential of from 1,000 to 2,000 volts. The lower end of the anode 6 may be operated at a positive potential with respect to the cathode 3 of approximately 100 volts. The anode 'l may be operated at a positive potential with respect to the cathode 3 of approximately 6,000 volts.

It Will be evident to those skilled in the art It Will be that the shape of the electrical concentrating eld developed by the rst anode provided by the present invention can be varied Within Wide limits by varying the shape of opening in the anode, the axial distribution of its electrical resistance and the voltage drop developed across the anode, either alone or in combination.

While there has been described what is at present considered the preferred embodiment of the invention, it Will be obvious to those skilled in the art that various changes and modications may be made therein Without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall Within the true spirit and scope of the invention.

What is claimed is:

1. In an electron gun, means for concentrating the flow of electrons comprising a unitary tubular anode member extending in the direction of said flow and having substantial non-uniform electrical resistance in said direction, and means for producing a current flow through said anode member to produce a voltage gradient across said anode means in the direction of said electron flow for developing a nonuniform electrical eld in the path of said flow to concentrate the electrons therein.

2. In an electron gun, means for concentrating the oW of electrons comprising a unitary tubular anode member extending in the direction of said ovv and consisting of material having substantial uniform electrical resistivity, the cross section of said anode varying in an axial direction, and means for producing a current flow through said anode member to produce a voltage gradient across said anode means in the direction of said electron flow for developing a nonuniform electrical field in the path of said lovv to concentrate the electrons therein.

3. In an electron gun, means for concentrating the oW of electrons comprising a unitary tubular anode member extending in the direction of said flow and having substantial electrical resistivity, the value of said resistivity varying in an axial direction, and means for producing a current ovv through said anode member to produce a voltage gradient across said anode means in the direction of said electron flow for developing a nonuniform electrical eld in the path of said flow to concentrate the electrons therein.

BERNARD C. GARDNER. 

